Method of Clustering Devices in Wireless Communication Network

ABSTRACT

In a wireless communication network ( 200 ) comprising a base station ( 210 ) and a plurality of remote terminals ( 220 ), the plurality of remote terminals ( 220 ) are divided into a plurality of clusters ( 230 ) for communication with the base station ( 210 ), and each of the remote terminals ( 220 ) is assigned to a cluster ( 230 ) based on at least one characteristic, measured by one or more of the remote terminals ( 220 ), of one or more external signals transmitted by one or more external terrestrial transmitters ( 250 ) not associated with the communication network ( 200 ). Accordingly: a parameter of a communication between the base station ( 210 ) and each remote terminal ( 220 ) may be selected according to the cluster ( 230 ) to which each remote terminal ( 220 ) belongs; remote terminals ( 220 ) within a cluster ( 230 ) may be enabled to communicate directly with each other; and/or remote terminals ( 220 ) may be selected to perform frequency spectrum profile measurements of the frequency band used by the communication network ( 200 ) according to the clusters ( 230 ) to which they are assigned.

This application claims the benefit of U.S. provisional application Ser. No. 60/718,127 filed Sep. 16, 2005, which is incorporated herein in whole by reference.

This invention pertains to the field of wireless communication networks, and more particularly to a method of clustering devices in a wireless communication network.

In the United States, the Federal Communications Commission (FCC) has recently released a proposed rulemaking to allow unlicensed wireless communication networks to operate on certain bands presently utilized by other, existing (“incumbent”), radio services, such as broadcast television. The FCC proposed standards to prevent the unlicensed wireless network's transmitting devices from interfering with the incumbent radio services. For example, these unlicensed transmitting devices are required to vacate any channel within a short time period (e.g., a few seconds) after an incumbent transmitter begins operating. One method of insuring that a transmitting device of an unlicensed wireless network vacates a channel when required to do so is for the device to periodically stop transmitting and to “listen” for incumbent transmitters by checking all channels within their operating band(s) for the presence of any transmissions from incumbent transmitters. If the device detects the presence of any incumbent radio transmissions, the device is then required to take appropriate measures (e.g., change channels; reduce power; shut down; etc.) to insure that it does not interfere with the incumbent signal(s).

FIG. 1 shows an exemplary unlicensed wireless communication network 100 comprising a base station (BS) 110, and a plurality of remote terminals (RTs) 120. In one embodiment, wireless communication network 100 may be a Wireless Regional Area Networks (WRAN). One typical application is broadband service where RTs 120 are on the consumer side (e.g., broadband modems) while BS 110 belongs to the service provider and services many RTs 120.

RTs 120 may be fixed or mobile devices. Typically, wireless communication network 100 may have as many as 100 or more RTs operating with BS 110. As shown in FIG. 1, in general there may be one or more external transmitters 150 (e.g., incumbent television transmitters) not associated with communication network 100 transmitting radio signal(s) in the same general geographical area as wireless communication network 100. Furthermore, new external transmitters 150 also may begin transmitting at any time, and these new transmitters are also considered incumbents so that their signals must be protected from interference by transmissions from any of RTs 120 or BS 110.

If BS 110 and all of the RTs 120 are required to periodically stop transmitting and listen for incumbent transmitters on every possible channel in order to meet the channel vacation requirements, the time required for this checking may be considerable and the frequency may be often, and this can significantly decrease the availability of wireless communication network 100.

Furthermore, wireless communication network 100 may operate over an area with a diameter on the order of tens of miles. So it is possible that a first group of RTs 120 may be located many miles closer to an incumbent external transmitter 150 than a second group of RTs 120. In that case, communication on one or more channels may be forbidden for the RTs 120 in the first group in order to protect the signal of the incumbent external transmitter 150, but communication on these same channels may be permissible for the second group of RTs 120 that are located many miles away from incumbent external transmitter 150. Conversely, the second group of RTs 120 may be located many miles closer than the first group of RTs 120 to a different, second incumbent external transmitter 150, so that communication on one or more different channels may be permissible for the first group of RTs 120, but forbidden for the second group of RTs 120. Even though BS 110 may know the locations and frequencies of all of the incumbent external transmitters 150 in its operating area, in general BS 110 has no convenient way of knowing which RTs 120 are located near which incumbent external transmitter 150. In that case, it may be forced to disable communication with all of the RTs 120 on all of the channels on which any of the incumbent external transmitters 150 are operating. This reduces the efficiency and data capacity of the network.

Additionally, in some cases it would be desirable, and would increase communication efficiency, for RTs 120 that are located in close proximity to each other to be able to communicate with each other directly, without passing data or messages through BS 110. However, if the BS 110 and RTs 120 have no convenient way of knowing which RTs 120 are located in close proximity, it is not practical to enable such direct communications.

Accordingly, it would be desirable to provide a method and means of grouping together remote terminals in a communication network that permits efficient assignment of resources for measuring the frequency spectrum profile of a frequency band used by the communication network. It would also be desirable to provide a method and means of grouping together remote terminals in a communication network that permits a base station to select and tailor one or more parameters of its communication with a remote terminal based on one or more common characteristics of the group to which the remote terminal belongs. It would further be desirable to provide a method and means of grouping together remote terminals in a communication network that facilitates direct communication between remote terminals that are in close geographical proximity to each other. It would still further be desirable to provide a system and method of determining the locations of fixed and mobile remote terminals in a wireless communication network.

In one aspect of the invention, in a wireless communication network comprising a base station and a plurality of remote terminals, a method of communication comprises dividing the plurality of remote terminals into a plurality of clusters for communication with the base station; assigning each of the remote terminals to one of the clusters based on at least one characteristic, measured by one or more of the remote terminals, of one or more external signals transmitted by one or more external terrestrial transmitters not associated with the wireless communication network; and selecting at least one parameter of a communication between the base station and each remote terminal according to a cluster to which each remote terminal belongs.

In another aspect of the invention, in a wireless communication network comprising a base station and a plurality of remote terminals, a method of communication comprises determining a location of each of the plurality of remote terminals with respect to the base station; dividing the plurality of remote terminals into a plurality of clusters for communication with the base station; assigning each of the remote terminals to one of the clusters based on the determined location of each remote terminal so as to group remote terminals together in each cluster according to their proximity to each other; and selecting at least one parameter of a communication between the base station and each remote terminal according to a cluster to which each remote terminal belongs.

In a further aspect of the invention, in a wireless communication network comprising a base station and a plurality of remote terminals, a method of determining a location of each of the plurality of remote terminals with respect to the base station comprises: (a) determining a distance, d12, between the base station and the remote terminal, based on a turnaround time interval, t12, for a token to be transmitted roundtrip between the base station and the remote terminal; (b) determining a time of arrival, t1, at the base station of a sync signal included in an external signal transmitted by an external terrestrial transmitter not associated with the wireless communication network and located at a known location; (c) determining a time interval, t₀₂, for the external signal to travel from the external terrestrial transmitter to the remote terminal using: (1) a known distance d₀₁ between the base station and the external terrestrial transmitter, (2) the time of arrival t1, and (3) a time of arrival, t₂, at the remote terminal of the sync signal included in the external signal transmitted by the external terrestrial transmitter not associated with the wireless communication network; (d) determining a distance, d₀₂, between the remote terminal and the known location of the external terrestrial transmitter, based on the time interval t₀₂; and (e) determining the location of the remote terminal using: (1) the distances d₁₂ and d₀₂, (2) the known location of the external terrestrial transmitter, and (3) a known location of the base station.

In still another aspect of the invention, in a wireless communication network comprising a base station and a plurality of remote terminals, a method of communication comprises dividing the plurality of remote terminals into a plurality of clusters for communication with the base station; assigning each of the remote terminals to one of the clusters based on at least one characteristic, measured by one or more of the remote terminals, of one or more external signals transmitted by one or more external terrestrial transmitters not associated with the wireless communication network; and enabling each remote terminal to communicate data directly with other remote terminals in its assigned cluster without passing the data through the base station.

In yet another aspect of the invention, in a wireless communication network comprising a base station and a plurality of remote terminals, a method of communication comprises dividing the plurality of remote terminals into a plurality of clusters for communication with the base station; assigning each of the remote terminals to one of the clusters based on at least one characteristic, measured by one or more of the remote terminals, of one or more external signals transmitted by one or more external terrestrial transmitters not associated with the wireless communication network; and selecting which ones among the plurality of remote terminals will perform frequency spectrum profile measurements of a frequency band used by the external terrestrial transmitters not associated with the wireless communication network, according to the clusters to which they are assigned.

FIG. 1 shows a wireless communication network;

FIG. 2 illustrates a wireless communication network including remote terminals divided into clusters;

FIG. 3 shows a diagram for explaining a method of determining the location of a remote terminal in a wireless communication network;

FIG. 4 illustrates a wireless communication network where remote terminals are divided into clusters based on geographical proximity to each other;

FIG. 5 illustrates a frequency spectrum profile measurement of incumbent transmissions in a frequency band used by a communication network; and

FIG. 6 shows a flowchart of a method of dividing remote terminals into clusters, and assigning the remote terminals to the clusters, in a wireless communication network.

While various principles and features of the methods and systems described below can be applied to a variety of communication systems, for illustration purposes the exemplary embodiments below will be described in the context of an unlicensed wireless communication network, such as that described above, that operates in one or more frequency bands that are populated with incumbent transmitters. Of course, the scope of the invention is defined by the claims appended hereto, and is not limited by the particular embodiments described below. Furthermore, as used herein, the term “an external terrestrial transmitter not associated with the wireless communication network” refers to any terrestrial radio transmitter that transmits its signal independently of the operation of the wireless communication network, for example: a terrestrial analog or digital television broadcast transmitter; a television relay transmitter; a terrestrial commercial radio broadcast transmitter; a radio repeater in the public service or amateur radio bands; etc.

Disclosed herein is a method of communication for a wireless communication network comprising a base station and a plurality of remote terminals. The method divides the plurality of remote terminals into a plurality of clusters for communication with the base station, and assigns each of the remote terminals to one of the clusters.

FIG. 2 illustrates a wireless communication network 200 including a base station (BS) 210 and a plurality of remote terminals (RTs) 220 divided into clusters 230.

As described in further detail below, each of the RTs 220 is assigned to one of the clusters 230 based on at least one characteristic, measured by one or more of the RTs 220, of one or more external signals transmitted by one or more external terrestrial transmitters 250 not associated with the wireless communication network 200.

In one embodiment, the measured characteristic is a time of arrival at an RT 220 of a sync signal included in an external signal transmitted by the external terrestrial transmitter 250. For example, where the external terrestrial transmitter 250 is a digital television (DTV) transmitter, the sync signal may be a field sync sequence in the DTV broadcast signal. In that case, the measured time of arrival of the sync sequence at RT 220 is used to calculate the location of the RT 220, which is in turn used to assign RT 220 to a particular cluster 230.

In another embodiment, the measured characteristic is “profile” of incumbent transmissions from all of the external terrestrial transmitters 250 that are received at each of the RTs 220. Beneficially, the incumbent profile may be a frequency spectrum profile, measured at each of the RTs 220, produced by the external signals from the external terrestrial transmitters 250. In that case, RTs 220 are assigned to clusters 230 in order to group together in each cluster 230 RTs 220 having similar incumbent (e.g., frequency spectrum) profiles.

In accordance with a first embodiment, FIG. 3 shows a diagram for explaining a method of determining the location of a remote terminal in a wireless communication network based on a time of arrival of an external signal transmitted by one or more external terrestrial transmitters not associated with the wireless communication network. FIG. 3 shows a base station (BS) 210, a remote terminal (RT) 220, and an external terrestrial transmitter 250 (e.g., a terrestrial broadcast television (TV) transmitter) not associated with the wireless communication network 200.

The location (x₁, y₁) of BS 210 is assumed to be known. The location (x₀, y₀) of external terrestrial transmitter 250 is also assumed to be known (a record of the location of TV transmitters in the United States is maintained by the FCC). Thus, the distance d₀₁ between TV transmitter 250 and BS 210 can be calculated and stored in BS 210. Additionally, the locations of BS 210 and TV transmitter 250 can be separately stored in BS 210.

Additionally, BS 210 may determine the distance d₁₂ between the RT 220 and itself in the following way. First, BS 210 transmits a token to RT 220 and requests that RT 220 respond back to BS 210. The turnaround time, t_(RT), to receive the response from RT 220, minus any processing time, can be used to calculate the distance d₁₂ between the BS 210 and RT 220 according to the following equation:

$\begin{matrix} {d_{12} = {c*\frac{t_{RT}}{2}}} & (1) \end{matrix}$

where c is the speed of light.

Next, the distance d₀₂ between the TV transmitter 250 and RT 220 is determined as follows.

A terrestrial television broadcast signal typically contains a known synchronization signal. For example, in the United States a terrestrial digital television (DTV) broadcast signal has a certain repetitive structure. A terrestrial DTV transmitter in the United States transmits a known signal (called a “frame sync”) every 24.2 ms.

This known signal can be used to compute the distance d₀₂ between TV station 250 and RT 220, as follows. First, BS 210 instructs ST 220 to search for the sync sequence in a television signal transmitted by TV transmitter 250. The time of arrival, t₂, of the sync sequence at RT 220 is determined. Meanwhile, BS 210 also searches for the sync sequence in the TV signal transmitted by TV transmitter 250, and records the time of arrival, t₁, of the sync sequence at its location. In that case, the time interval, t₀₂, needed for the TV signal to travel from TV transmitter 250 to RT 220, can be calculated as:

t ₀₂ =t ₂−(t ₁ −d ₀₁ /c)  (2)

Once t₀₂ is known, then d₀₂ can be calculated as:

d ₀₂ =c*t ₀₂  (3)

Now that d₀₂ and d₁₂ have been calculated as described above, one can determine the location (x₂, y₂) of RT 220 by solving the following pair of simultaneous equations:

d ₀₂ ²=(x ₀ −x ₂)²+(y ₀ −y ₂)²

d ₁₂ ²=(x ₁ −x ₂)+(y ₁ −y ₂)  (4)

Except for x₂ and y₂, all of the other variables in the equation pair (4) are known. So by simultaneously solving the equation pair, the location (x₂, y₂) of RT 220 can be found.

Meanwhile, a number of factors may negatively impact the accuracy of the location determination method described above. For example, multipath and clock mismatches may affect the accuracy of the time-of-arrival measurements. Fortunately, for broadband wireless communication network applications, a high degree of accuracy is not required. In such an application, BS 210 only needs to know the approximate location of RT 220 so that it can group RTs 220 accordingly. In those cases, the method described above is typically satisfactory.

In those circumstances where a more accurate determination of the location of RT 220 is needed, the accuracy can also be greatly improved by repeating the above-described procedure for two or more different external terrestrial transmitters 250 (e.g., TV transmitters) not associated with the wireless communication network 200, and then averaging the results to more accurately determine the location of RT 220.

Furthermore, if signals transmitted by two or more different external terrestrial transmitters 250 (e.g., TV transmitters) not associated with wireless communication network 200 are available, then the location of RT 220 in three-dimensional space (x₂, y₂, z₂) can also be calculated by solving the following equation set:

d ₀₂ ²=(x ₀ −x ₂)²+(y ₀ −y ₂)²+(z ₀ −z ₂)²

d ₁₂ ²=(x ₁ −x ₂)²+(y ₁ −y ₂)²+(z ₁−z₂)²

d ₂₃ ²=(x ₃ −x ₂)²+(y ₃ −y ₂)²+(z ₃ −z ₂)²  (5)

where d₂₃ is the distance between RT 220 and a second TV transmitter 250 determined using the procedure described above, (x₁, y₁, z₁) is the location of BS 210 in three-dimensional space, (x₀, y₀, z₀) is the location of the first TV transmitter 250 in three-dimensional space, and (x₃, y₃, z₃) is the location of the second TV transmitter 250 in three-dimensional space.

The procedures described above can be performed for all RTs 220 in wireless communication network 200 so that BS 210 learns the locations of all of the RTs 220.

The performance of an unlicensed wireless communication network operating in a frequency band utilized by one or more incumbent transmitters can be enhanced if the locations of the remote terminals of the wireless communication network are known. When the locations are known, a base station can divide the remote terminals into a plurality of clusters, and assign the remote terminals to the clusters so as to group remote terminals together in each cluster according to their proximity to each other. In that case, techniques such as group scheduling or multiple antenna diversity can be employed. Remote terminals in the same geographical area can be made to share the same directionality thereby improving capacity as well as performance.

FIG. 4 illustrates a wireless communication network 200 comprising BS 210 and RTs 220, where RTs 220 have been divided into clusters 230, and each RT 220 is assigned to one of the clusters 230 so as to group RTs 220 together in each cluster 2430 according to their proximity to each other.

By clustering RTs 220 together according to their geographical proximity, BS 210 can do one or more of the following.

-   -   BS 210 can select at least one parameter of communication         between BS 210 and each RT 220 according to the particular         cluster 230 to which that RT 220 belongs. For example, BS 210         may select different modulation and/or error correction coding         formats for different clusters 230 of RTs 220 depending upon the         general location of the cluster 230. That is, BS 210 may select         a more robust coding/modulation format for clusters 230 of RTs         220 that are distant from BS 210, or for clusters 230 of RTs 220         that are located close to an external terrestrial transmitter         250 not associated with the wireless communication network 200,         and which therefore experience increased interference.         Furthermore, BS 210 may optimize the guard interval when a         multi-carrier scheme such as orthogonal frequency division         multiplexing (OFDM) is employed, according to the expected         multipath delay spread of a particular cluster 230. Thus,         clustering allows BS 210 to tailor one or more parameters of its         communication with an RT 220 based on one or more common         characteristics of the cluster 230 to which the RT 220 belongs.     -   BS 210 can use a directional antenna in combination with         techniques such as space division multiplexing between clusters         230. This can increase the overall capacity of the wireless         communication network 200, since RTs 220 that are not in the         same cluster 230 can transmit and receive at the same time with         little interference. Also, BS 210 may use different frequency         channels to communicate with different clusters 230 of RTs 220         depending upon the relative locations of incumbent transmitters         250. That is, it may be possible for BS 210 to use a first         frequency channel for communication with a first cluster 230,         while it is not permitted to use that same first frequency         channel for communication with a second cluster 230 because of         the proximity of the second cluster 230 to an incumbent         transmitter 250 operating on the first frequency channel. At the         same time, BS 210 may be able to use a second frequency channel         to communicate with the second cluster 230, while it is not         permitted to use that channel for communication with the first         cluster 230 because of the proximity of the first cluster 230 to         a second incumbent transmitter 250 operating on the second         frequency channel. Thus, clustering allows BS 210 to more         efficiently utilize its communication resources in communicating         with a plurality of RTs 220.     -   BS 210 can schedule RTs 220 in a cluster 230 to communicate         directly with each other, without having to pass messages or         data through BS 210. This can produce a multi-sensor network         that can be used for applications other than broadband service.

Although for ease of explanation, in the discussion above the external terrestrial transmitter 250 was described in terms of a terrestrial broadcast television (TV) transmitter, in practice external terrestrial transmitter 250 can be any external terrestrial transmitter that transmits a signal including some sync or other feature of pattern that is amenable to time-of-arrival detection and whose location is known to BS 210. In one embodiment, external terrestrial transmitter 250 comprises a dedicated beacon transmitter transmitting a signal which can be used for clustering together RTs 220 in wireless communication network 200.

Although a process of clustering remote terminals in a wireless communication network has been described above based on determining a geographical location of each remote terminal, in another embodiment remote terminals are assigned to clusters according to an incumbent profile measured at each of the remote terminals produced by one or more external signals transmitted by one or more external terrestrial transmitters not associated with the wireless communication network. In that case, the location of the external over-the-air transmitter need not be known, and remote terminals are assigned to clusters in order to group together in each cluster remote terminals having similar incumbent profiles.

According to this embodiment, each RT 220 makes measurements in each incumbent (e.g., TV) channel of external signals (e.g., TV signals) transmitted by one or more external terrestrial transmitters 250 not associated with the wireless communication network 200. The incumbent profile measurement can be a simple RF signal strength measurement of the frequency spectrum used by wireless communication network 200. Alternatively, more sophisticated measurements may be made based on the detection of a feature of each external signal to provide greater robustness to multipath. In the latter case, beneficially the strength of the detected feature is used. For example, if the incumbent transmitter 250 is nearby (or transmitting at high power), its value will be high, and vice versa. Based on these measurements, each RT 220 constructs an incumbent profile. This incumbent profile is then disseminated to BS 210 (or its proxy) for clustering, as described in further detail below. This process can be repeated periodically.

FIG. 5 illustrates a frequency spectrum profile measurement, made by an RT 220, of incumbent transmissions in a frequency band used by wireless communication network. 200.

Next, an algorithm is described for clustering together RTs 220 having similar incumbent profiles will be described with respect to the flowchart of FIG. 6.

At the outset, we define a number of variables as follows:

n=the number of RTs 220 in wireless communication network 200;

f=total number of frequency channels used by wireless communication network 200 that may include an external signal transmitted by an external terrestrial transmitter 250;

k=number of clusters 230 into which the RTs 220 are divided;

i=an index for each RT 220, where 1≦i≦n; and

j=an index of each cluster 230, where 1≦i≦k;

x_(i) =a measurement vector for RT 2201, of size 1*f;

J=a scalar objective function to be minimized;

J*=maximum allowed value for scalar objective function (an input value);

k*=minimum number of clusters 230 required for J≦J* (an output value).

Turning again to FIG. 6, the algorithm proceeds as follows.

In a step 610, the number of clusters 230 is set to two (2) (k=2).

Meanwhile, in a step 620 each of the n RTs 220 measures a frequency spectrum profile at its location, as described above, to produce a measurement vector, x₁ .

Then, in a step 630, k of the measurement vectors x_(i) of the RTs 220 are randomly assigned as trial mean measurement vectors, m_(j) , for the k clusters 230. These k trial mean measurement vectors m_(j) serve as initial guesses as to the actual mean measurement vectors for the k clusters 230.

Next, in a step 640, for each RT 2201, it is determined which one of the mean measurement vectors m_(j) is closest to its measurement vector x_(i) , and the RT 2201 is then assigned to the cluster, j, as a trial assignment.

After all of the RTs 220 have been assigned to one of the k clusters 230, in a step 650 an “updated” mean measurement vector m_(j) is calculated for each cluster 230 j using the measurement vectors x_(i) ^((j)) for all of the RTs 220 i in that cluster 230 j.

Steps 640 and 650 are repeated until there is no further change in the values of the mean measurement vectors m_(j) .

Next, in a step 660, the scalar objective function to be minimized, J, is calculated using the mean measurement vectors m_(j) for each cluster 230 j and all of the measurement vectors x_(i) ^((j)) .

In a step 670, the scalar objective function to be minimized, J, is compared to the maximum allowed value for the scalar objective function, J*. J* is a pre-selected value based on target performance criteria for the wireless communication network 200, and may be determined through operational experience.

If J>J*, then in a step 680, the algorithm increments k by one, and returns to step 630 above, and steps 630-670 are repeated.

If J≦J*, then the algorithm ends. At that point, k is equal to k*, and the RTs 220 are assigned to the k* clusters 230 so as to group together in each cluster 230 remote terminals 220 having similar incumbent profiles.

Mathematically, one beneficial selection for the scalar function, J, is:

$\begin{matrix} {J = {\sum\limits_{j = 1}^{k}{\sum\limits_{i = 1}^{n}{{\overset{\_}{x_{i}^{(j)}} - \overset{\_}{m_{j}}}}^{2}}}} & (6) \end{matrix}$

where ∥ x_(i) ^((j)) − m _(j)∥² indicates the distance between the measurement vector x_(i) ^((j)) of RT 220 i, tentatively assigned to cluster 230 j, and its cluster mean m_(j) , in feature space.

There are a number of advantages of clustering remote terminals. Some of these advantages relate to sharing the spectrum measurement responsibilities within the wireless communication network, and/or to more efficient dissemination of measurement information. If all the remote terminals measure all the channels and disseminate this information over the wireless communication network, the load on the network could be significant. By decimating the number of measurements made, the dissemination overhead is significantly reduced.

In this regard, it is noted that the frequency with which a given channel must be measured for occupation by an incumbent transmitter depends not on the duty cycle of the incumbent transmitter (which may be of the order of a day), but rather on the vacation time period, which may be of the order of a few seconds. The vacation time period is defined as the time period by which the wireless communication network must vacate a channel after an incumbent transmitter begins transmitting on that channel. When the vacation time period is small, unless information dissemination overhead is efficiently managed, it could become significant part of the total available radio resources. This is especially true if contention-based access mechanisms are used to disseminate measurement information.

However, once remote terminals are clustered together based on similar incumbent profiles, each RT does not have to make repeated measurement of the entire available spectrum. The base station (or its proxy) can make the optimal distribution of measurements within a network, which involves the following trading off. If too few remote terminals in the wireless communication network make measurements, an incumbent transmitter might be missed. On the other hand, if each remote terminal searches every channel once each vacation time period, the total amount of time it takes to determine which channels are available could be very large. The above-described approach of clustering provides an intelligent tool to make such a trade-off.

While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims. 

1. In a wireless communication network (200) comprising a base station (210) and a plurality of remote terminals (220), a method of communication, comprising: dividing the plurality of remote terminals (220) into a plurality of clusters (230) for communication with the base station (210); assigning each of the remote terminals (220) to one of the clusters (230) based on at least one characteristic, measured by one or more of the remote terminals (220), of one or more external signals transmitted by one or more external terrestrial transmitters (250) not associated with the wireless communication network (200); and performing at least one of the following three operations: (1) selecting, according to a cluster (230) to which each remote terminal (220) belongs, at least one parameter of a communication between the base station (210) and each remote terminal (220); (2) selecting, according to the clusters (230) to which they are assigned, which ones among the plurality of remote terminals (220) will perform frequency spectrum profile measurements of a frequency band used by the external terrestrial transmitters (250) not associated with the wireless communication network (200); and (3) selecting, according to the clusters (230) to which they are assigned, which ones among the plurality of remote terminals (220) will transmit to the base station (210) frequency spectrum profile measurements of a frequency band used by the external terrestrial transmitters (250) not associated with the wireless communication network (200).
 2. The method of claim 1, wherein the at least one characteristic measured by the one or more remote terminals (220) comprises a frequency spectrum profile measured at each of the remote terminals (220) produced by the one or more external signals transmitted by the one or more external terrestrial transmitters (250) not associated with the wireless communication network (200).
 3. The method of claim 2, wherein the frequency spectrum profile comprises a measurement vector, x_(i) , of size 1*f where f is a number of channels assigned for defining the external signals transmitted by the one or more external terrestrial transmitters (250), where i is an index number of a corresponding remote terminal (220), and where j is an index number of a cluster (230).
 4. The method of claim 3, wherein the plurality of remote terminals (220) is n, and wherein dividing the n remote terminals (220) into a plurality of clusters (230) for communication with the base station (210) and assigning each of the n remote terminals (220) to one of the clusters (230) based on the measured characteristic of the one or more external signals comprises: (610) establishing a trial value of k=2 clusters (230); (630) assigning k of the n measurement vectors of the n remote terminals (220) as trial mean measurement vectors, m_(j) , for each of the k clusters (230); (640) for each of the n remote terminals (220), determining which of the trial mean measurement vectors, m_(j) , is closest to its measurement vector x_(i) and tentatively assigning the remote terminal (220) to the corresponding cluster (230) j as a tentative assignment; (650) for each cluster (230), j, calculating a new mean measurement vector m_(j) using the measurement vectors for all of the remote terminals (220) tentatively assigned to the cluster (230) j in step (640); repeating steps (640) and (650) until there is no change in the values of the mean measurement vectors m_(j) ; (660) computing a total vector distance, J, between the measurement vectors x_(i) for each of the n remote terminals (220) and the mean vector m_(j) for the cluster (230) j to which the remote terminal (220) was last tentatively assigned in step (640); (670) comparing J to a maximum allowed value J*; (680) when J is greater than the maximum allowed value J*; incrementing k by 1 and repeating steps (630) through (670) until J is less than or equal to the maximum allowed value J*; and when J is less than or equal to the maximum allowed value J*; using the trial value of k as a number of clusters (230) into which the n remote terminals (220) are divided, and using the tentative assignments of step (c) to assign each of the n remote terminals (220) to one of the k clusters (230).
 5. The method of claim 1, wherein the at least one parameter of the communication between the base station (210) and the remote terminal (220) that is selected according to a cluster (230) to which the remote terminal (220) belongs comprises at least one selected from the group consisting of: an error correction code, a modulation format, a guard interval between adjacent transmitted symbols, and one or more frequency channels.
 6. The method of claim 1, wherein the at least one characteristic of the one or more external signals measured by the one or more remote terminals (220) comprises a time of arrival of a sync signal included in at least one of the one or more external signals transmitted by the one or more external terrestrial transmitters (250) not associated with the wireless communication network (200).
 7. The method of claim 6, wherein the sync signal is a field sync sequence in a digital television (DTV) broadcast signal.
 8. The method of claim 1, wherein the one or more external signals transmitted by the one or more external terrestrial transmitters (250) not associated with the wireless communication network (200) includes a television broadcast signal in a frequency channel in which the wireless communication network (200) operates.
 9. In a wireless communication network (200) comprising a base station (210) and a plurality of remote terminals (220), a method of communication, comprising: determining a location of each of the plurality of remote terminals (220) with respect to the base station (210); dividing the plurality of remote terminals (220) into a plurality of clusters (230) for communication with the base station (210); assigning each of the remote terminals (220) to one of the clusters (230) based on the determined location of each remote terminal (220) so as to group remote terminals (220) together in each cluster (230) according to their proximity to each other; and selecting at least one parameter of a communication between the base station (210) and each remote terminal (220) according to a cluster (230) to which each remote terminal (220) belongs.
 10. The method of claim 9, wherein determining a location of each of the plurality of remote terminals (220) with respect to the base station (210), comprises, for each remote terminal (220): determining a distance, d₁₂, between the base station (210) and the remote terminal (220); determining a distance, d₀₂, between the remote terminal (220) and a known location of a television broadcast antenna; and solving a pair of simultaneous equations using: (1) the distances d₁₂ and d₀₂, (2) the known location of the television broadcast antenna, and (3) a known location of the base station (210), to determine the location of the remote terminal (220).
 11. The method of claim 10, wherein determining the distance, d₁₂, between the base station (210) and the remote terminal (220) comprises: measuring a turnaround time interval, t₁₂, for a token to be transmitted roundtrip between the base station (210) and the remote terminal (220); and determining the distance, d₁₂, from the turnaround time interval, t₁₂.
 12. The method of claim 10, wherein determining the distance, d₀₂, between the known location of the television broadcast antenna and the remote terminal (220), comprises: determining a time of arrival, t₁, at the base station (210) of a sync signal included in a television signal transmitted by the television broadcast antenna; determining a time of arrival, t₂, at the remote terminal (220) of the sync signal included in the television signal transmitted by the television broadcast antenna; determining a time interval, to 2, for the television signal to travel from the television broadcast antenna to the remote terminal (220) using the times of arrival t₁ and t₂ and a known distance d₀₁ between the base station (210) and the television broadcast antenna; and determining d₀₂ according to the equation d₀₂=t₀₂*c, where c is the speed of light.
 13. The method of claim 12, wherein the television signal is in a frequency channel in which the wireless communication network (200) operates.
 14. The method of claim 9, wherein the at least one parameter of the communication between the base station (210) and the remote terminal (220) that is selected according to a cluster (230) to which the remote terminal (220) belongs comprises at least one selected from the group consisting of: an error correction code, a modulation format, a guard interval between adjacent transmitted symbols, and one or more frequency channels.
 15. In a wireless communication network (200) comprising a base station (210) and a plurality of remote terminals (220), a method of determining a location of each of the plurality of remote terminals (220) with respect to the base station (210), comprising: (a) determining a distance, d₁₂, between the base station (210) and the remote terminal (220), based on a turnaround time interval, t₁₂, for a token to be transmitted roundtrip between the base station (210) and the remote terminal (220); (b) determining a time of arrival, t₁, at the base station (210) of a sync signal included in an external signal transmitted by an external terrestrial transmitter (250) located at a known location; (c) determining a time interval, t₀₂, for the external signal to travel from the external terrestrial transmitter (250) to the remote terminal (220) using: (1) a known distance d₀₁ between the base station (210) and the external terrestrial transmitter (250), (2) the time of arrival t₁, and (3) a time of arrival, t₂, at the remote terminal (220) of the sync signal included in the external signal transmitted by the external terrestrial transmitter (250); (d) determining a distance, d₀₂, between the remote terminal (220) and the known location of the external terrestrial transmitter (250), based on the time interval to 2; and (e) determining the location of the remote terminal (220) using: (1) the distances d₁₂ and d₀₂, (2) the known location of the external terrestrial transmitter (250), and (3) a known location of the base station (210).
 16. The method of claim 15, wherein the external terrestrial transmitter (250) is a television broadcast transmitter (250) and the external signal is a television signal in a frequency channel in which the wireless communication network (200) operates.
 17. The method of claim 15, further comprising: (f) determining a time of arrival, t₃, at the base station (210) of a sync signal included in a second external signal transmitted by a second external terrestrial transmitter (250) not associated with the wireless communication network (200) located at a known location; (g) determining a time interval, t₂₃, for the external signal to travel from the second external terrestrial transmitter (250) to the remote terminal (220) using: (1) the known distance d₁₃ between the base station (210) and the second external terrestrial transmitter (250), (2) the time of arrival t₃, and (3) a time of arrival, t₄, at the remote terminal (220) of the sync signal included in the second external signal transmitted by the second external terrestrial transmitter (250) not associated with the wireless communication network (200); (h) determining a distance, d₂₃, between the remote terminal (220) and a known location of the second external terrestrial transmitter (250), based on the time interval t₂₃; (i) determining the location of the remote terminal (220) using: (1) the distances d₁₂ and d₂₃, (2) the known location of the second external terrestrial transmitter (250), and (3) the known location of the base station (210); and (j) averaging the locations produced in steps (e) and (i) using the first and second external signals transmitted by the first and second external terrestrial transmitters (250) to more accurately determine the location of the remote terminal (220).
 18. The method of claim 15, further comprising: determining a time of arrival, t₃, at the base station (210) of a sync signal included in a second external signal transmitted by a second external terrestrial transmitter (250) not associated with the wireless communication network (200), located at a second known location; determining a time interval, t₂₃, for the external signal to travel from the second external terrestrial transmitter (250) to the remote terminal (220) using: (1) the known distance d₁₃ between the base station (210) and the second external terrestrial transmitter (250), (2) the time of arrival t₃, and (3) a time of arrival, t₄, at the remote terminal (220) of the sync signal included in the second external signal transmitted by the second external terrestrial transmitter (250) not associated with the wireless communication network (200); and determining a distance, d₂₃, from the known location of second external terrestrial transmitter (250) to the remote terminal (220) based on the time interval t₁₃, wherein determining the location of the remote terminal (220) using: (1) the distances d₁₂ and d₀₂, (2) the known location of the external terrestrial transmitter (250); and (3) the known location of the base station (210), further comprises determining the location of the remote terminal (220) in three dimensions by further using (4) the known location of the second external terrestrial transmitter (250); and (5) the distance d₂₃.
 19. In a wireless communication network (200) comprising a base station (210) and a plurality of remote terminals (220), a method of communication, comprising: dividing the plurality of remote terminals (220) into a plurality of clusters (230) for communication with the base station (210); assigning each of the remote terminals (220) to one of the clusters (230) based on at least one characteristic, measured by one or more of the remote terminals (220), of one or more external signals transmitted by one or more external terrestrial transmitters (250) not associated with the wireless communication network (200); and enabling each remote terminal (220) to communicate data directly with other remote terminals (220) in its assigned cluster (230) without passing the data through the base station (210).
 20. In a wireless communication network (200) comprising a base station (210) and a plurality of remote terminals (220), a method of communication, comprising: dividing the plurality of remote terminals (220) into a plurality of clusters (230) for communication with the base station (210); assigning each of the remote terminals (220) to one of the clusters (230) based on at least one characteristic, measured by one or more of the remote terminals (220), of one or more external signals transmitted by one or more external terrestrial transmitters (250) not associated with the wireless communication network (200); and selecting which ones among the plurality of remote terminals (220) will perform frequency spectrum profile measurements of a frequency band used by the external terrestrial transmitters (250) not associated with the wireless communication network (200), according to the clusters (230) to which they are assigned. 